I also have noticed my CYA dropping. I did not know CYA can be lost with Slamming and 90 degree water. I do not feel crazy anymore!
I thought I was at 35cya at the beginning of season because test last year stated that. Then i added 2 3" pucks on 7/12/16, 3 3" pucks on 07/19, 34 ounces stabilizer powder on 07/21/16 and 8 more 3" pucks by the date of 08/09/16. I have checked on and off cya level at home when I have the test and pool store otherwise. I had to order more cya tests and today the dot barely disappeared at 28?? I just added 3 more pucks today only because I have them for vacations and at this point they are free. My pool consumed 3 128 oz liquid shock today. I have been slamming all summer with a clear pool that will not hold overnight oclt and has 2 cc. I check about 3 to 4 times a day and use liquid shock and have been aiming between 16 - 18. I am averaging 2 128 oz 10% liquid shock a day and sometimes 3. On vacation I had someone stay here and run pump 24/7 like I have all summer. Dump in 64 oz in morning, 64 oz around 3 and 128 oz at dark just to keep it from turning green and to simplify. I just wanted to share that I also have had CYA disappearing. All summer I have spent $70 every two weeks on liquid shock and purchase large taylor regents of 0871, 0870 powder, stabilizer in granular form, ammonia test from fish store, Cya tests and not to mention my electric bill. And while on vacation we had like 4 inches of rain. My skimmer is now at the top with water. Good luck sky dude. I always thought splash out, ammonia, leaks and draining water were the only ways CYA left the pool. So, with me, my cya has been too low due to constant slamming, therefore the sun is consuming too much of my chlorine and causing my efforts to slam to fall short. In my case I guess using the automatic chlorine feeder during my slam process would of helped. I was counting, writing it down, ppm of CYA I added and trying to be careful to not add too much.